Mitt Romney was fighting to keep his presidential ambitions on track after further revelations from a secretly recorded speech showed him dismissing any prospect of Middle East peace, describing Iran's leaders as "crazy people" and offering details on how terrorists could bomb Chicago.

The new excerpts came hours after Mr Romney was forced to give a hastily scheduled late-night press conference to explain why, at the same event in Florida, he had apparently dismissed half the US electorate as welfare dependents, concluding "My job is not to worry about those people".

The latest round of secretly recorded tapes raised fresh questions over Mr Romney's already weak foreign policy credentials, and completely over-shadowed attempts by the Republican candidate to 'reset' his US election campaign this week by focusing on policy specifics.

Asked how he would tackle the quest for Middle East peace if elected, Mr Romney told guests at the $50,000-a-plate fund-raiser in Boca Raton, Florida last May that he would essentially do nothing.

On Iran, Mr Romney raised the threat of an Iranian 'dirty bomb' without apparently realising that such a device would did not require enriched, fissile material of the kind Iran is accused of processing.

He offered his thoughts on what could be done with it saying: "If I were Iran, if I were Iran–a crazed fanatic, I'd say let's get a little fissile material to Hezbollah, have them carry it to Chicago or some other place, and then if anything goes wrong, or America starts acting up, we'll just say, "Guess what? Unless you stand down, why, we're going to let off a dirty bomb.

"I mean this is where we have–where America could be held up and blackmailed by Iran, by the mullahs, by crazy people. So we really don't have any option but to keep Iran from having a nuclear weapon," he added.

In the same speech Mr Romney, has a $250m fortune, joked that he would have a better chance of being elected if his father, who lived in Mexico in his youth, had actually been born to Mexicans. "It would be helpful to be Latino," he said, adding, in a reference to his fortune: "Frankly, I was born with a silver spoon in my mouth."

Mr Romney yesterday refused to back away from his remarks about low-income voters, insisting that he had simply been stating a political reality and recognising "the perspective of individuals who are not likely to support me".

"Those that are dependent on government and those that think government's job is to redistribute - I'm not going to get them," he told Fox News in his first interview since the footage emerged.

He said his comments at the fundraiser were not different from "the message I'm carrying in day in and day out" about the need to reduce the size of government and get it out of the way of private enterprise.

Mr Romney was not asked about his views on the Palestinians or Iran during the interview.

Commenting on the footage for the first time last night, Mr Obama accused Mr Romney of "writing off" a large section of the population. "If you want to be president, you have to work for everyone," he said in an interview on the Late Show With David Letterman.

"There are not a lot of people out there who think they're victims," the President added. "There are not a lot of people who think they’re entitled to something.”

With less than 50 days to go until polling day, the fallout from the tapes published by the Mother Jones website dominated the media landscape yesterday. Mr Romney was accused of harping on old social divisions between "makers and takers" with his remark that 47 per cent of Americans "pay no income tax" when experts pointed out that the majority of households did pay payroll taxes – the equivalent of Britain's national insurance.

Of the 18 per cent of households who paid neither federal income tax nor payroll tax, roughly half were elderly people, who were likely to have paid income tax for years before retiring, and are now a crucial segment of Mr Romney's likely support base.

Many Conservative commentators, already despairing at Mr Romney's clumsy response to last week's Middle East protests, did not spare him with some describing the footage as "potentially crippling".

"As a description of America today, Romney's comment is a country-club fantasy. It's what self-satisfied millionaires say to each other. It reinforces every negative view people have about Romney," wrote David Brooks, a leading conservative commentator in the New York Times.

David Frum, a White House speechwriter for Mr Bush, said: "If you're not running for president of all the country, you won't be elected president of any of it."

However, others cautioned that Mr Romney's remarks would appear less damaging out on the doorsteps of America than they first appeared in the television studios and on liberal-dominated social media networks.

Jeffrey Lord, a former Reagan aide and Republican party historian, said the remarks on welfare-dependents would strike a chord with many of America's hard-pressed middle classes.

"It's still unclear to me what impact this will have," he told The Telegraph, "A lot of the middle class who pay their taxes are going to listen to Romney and say 'yeah, damn right'.

White House spokesman Jay Carney responded to the Romney comments saying: "When you're president of the United States, you are president of all the people, not just the people who voted for you."